Until a color photo is obtained I cannot see why thy oil fill tube would not be engine color. No other part of the factory engine is unpainted. I had first heard about the possibility of it being 'terne plated' (which is a mix of lead and tin, like a plating) this year but was never shown any supporting evidence. And there is no reason to put that one part (the fill tube) though a plating process when paint would be more than adequate.
If you think the fill tube is tern plated, I have advocated this simple test: Go to a hardware and get a lead paint detector (it looks like a crayon). Scratch a small part and test for lead. Terne plating is up to 90% lead... if it comes back as indicating lead, then I am open to more discussion. Tern plating is still used today, but uses zinc, not lead... I think its called Terne 2 plating.
This is a good shot of a oil fill tube:
Clearly its painted, but black or GMC Olive is difficult (well, impossible) to tell. But the wear by the cap seems to show that paint is missing.... no matter what the color.
IIRC this is from Jun 1942 in Kentucky.. the photo caption is:
Fort Knox. Negro mechanics. A good ignition job on an army truck. This Negro soldier, who serves as truck driver and mechanic at Fort Knox, Kentucky, plays an important part in keeping army transport fleets in operation